There was a chance that the fragment had been handled at some point by a sales clerk who sold the pressure cooker or by somebody who retrieved the fragment. It was even possible that the piece had been blown clear through some innocent victim.

But there was also a chance that Tamerlan Tsarnaev’s wife had been more involved than she has led investigators to believe. She submitted to a DNA swab after some discussions between the authorities and her lawyer.

A positive match could lead to an arrest.

A negative would go a long way toward dispelling suspicions that she was somehow complicit.

This file image from a Federal Bureau of Investigation and Department of Homeland Security joint bulletin, shows the remains of a pressure cooker that the FBI says was part of one of the bombs that exploded during the Boston Marathon. (FBI via AP)

In the meantime, it is worth considering whether there is some psychic connection between the Marathon attack and a bombing last May. That previous attack killed 10 more than died in Boston, but it was on an ordinary workday in distant Makhachkala, and it received barely a mention in the American media.

One person on whom this bombing in the capital city of Dagestan undoubtedly made a big impression was Tamerlan Tsarnaev, who was visiting family there at the time. It is hard to imagine that people in Makhachkala spoke much of anything else in the immediate aftermath, especially after authorities announced that it had been a suicide attack carried out by siblings, a 25-year-old brother and his 19-year-old sister.

The Islamic militants believed to be behind the attack were still at large and the city was placed under a “special counter-terrorist regime” that could not have escaped Tamerlan’s attention. People were subject to being stopped and questioned at any time and liable to be detained if they were unable to explain satisfactorily their purpose for being out and about.

On May 19, police surrounded the Makhachkala hideout of the man said to be the mastermind of the attack, 19-year-old Mahmoud Mansur Nidal. He negotiated for the three women and a child who were with him to come out safely. He then refused to surrender and was killed in the ensuing shootout.

Both men had emigrated from Russia in their teens, Tamerlan from Dagestan to Cambridge, Plotnikov from Siberia to Toronto. Both had been highly regarded amateur boxers before becoming Islamic extremists. Plotnikov’s transformation had been all the more startling because his parents are Christian.

President Obama spoke movingly at a tribute to those killed and injured in the Boston bombings.

The question now is whether there was a female involved in the making of the bombs in Boston that got the attention of the whole world.

In September 2010, Plotnikov left a note on the kitchen table of his family's Toronto home saying he had jetted off to France for Ramadan. The family learned that he had in fact gone to Dagestan and asked the Russian authorities to dissuade him from joining up with the militants there.

That December, Plotnikov was interviewed by the security services. Novaya Gazeta reports that when he was asked who he had been communicating with online, the names he gave included Tamerlan Tsarnaev.

If that is so, it may well be what prompted the Russians to ask the FBI to conduct a check on Tamerlan. It also would explain why the Russians were not more specific, as they had not uncovered enough to warrant detaining Plotnikov even under their far more flexible guidelines. The Russians appear to have been satisfied when Plotnikov agreed to leave Dagestan.

But Plotnikov had no sooner reached Moscow than he headed back to Dagestan. He joined up with a band of insurgents and became known as “The Canadian.” He made a video in which he asked one of his comrades, “What do you do?”

“Terrorism,” the comrade replied. “I kill kafirs.”

Kafirs being nonbelievers, the enemy being much more than just Russians. Plotnikov used the term when taking a turn at speaking before the camera.

“I ask Allah that the next season he’ll give us the opportunity to kill as many kafirs as we can,” he said. “Just to shred them to pieces.”

Tamerlan also had begun speaking about kafirs when he came to visit his family in Dagestan in January 2012. He may or may not have met up with the fellow boxer with whom he reportedly had cyberchats two years before.

On July 14, Plotnikov and seven other insurgents were killed in a nighttime ambush in a forest outside the city. The security services came asking for Tamerlan at his father’s house in Makhachkala several days later.

The father reportedly told them that Tamerlan had returned to America. The security services reportedly did not believe him at first, especially since Tamerlan had applied for a new Russian passport at the end of June and had not yet picked it up. Authorities began checking the bus and train stations, as well as the airports.

Wittingly or no, Tamerlan was a step ahead of them, having flown to Moscow on July 16, two days after Plotnikov was killed. Tamerlan flew from there to New York on July 17. He continued on to Cambridge, where he had been living with his wife and young daughter, so far from Makhachkala that the bombing had passed without notice and nobody had heard of Nidal or Plotnikov.

By February, Tamerlan was assembling the makings of his own bombs with the intent of bringing jihadist violence to the Boston Marathon on Patriots' Day. These bombs were also to be planted by siblings, only both were male and neither was apparently ready to blow up himself along with his victims.

The question now is whether there was a female involved in the making of the bombs in Boston that got the attention of the whole world.